The present invention relates to an oil/water separation apparatus for removing oil from an oil-containing mixture. “Oil-containing mixture” as used here refers to oil-water mixtures, oil mixed into water, water mixed into oil, emulsified water discharged from a compressor or the like, and so on.
Drain waste liquids are discharged from the many compressors that are used in ordinary factories. These drain waste liquids mainly consist of emulsified, cloudy liquids, which are produced when the lubricating oil mists that drift throughout a device bond with water in the air. Once admixed, water and oil are not easily separated. These drain waste liquids, which contain 50 to 1000 ppm oil, need to have the oil removed before being released into the water system, for the sake of environmental protection.
Common types of apparatus for removing the oil from such oil-containing mixture include filtration membrane types, specific gravity separation types, adsorption types, and electrolysis types.
A filtration membrane type of apparatus makes use of an ultrafiltration membrane or a reverse osmosis membrane.
A specific gravity separation type of apparatus separates the oil from an oil-containing mixture by means of the difference in the specific gravity between oil and water. This type of apparatus is disclosed in Japanese Patent Application 2003-301776, which is a drain treatment apparatus comprising a separation unit and an evaporation unit that is connected to the separation unit. The separation unit utilizes a specific gravity differential to separate the oil in a drain waste liquid from an air compressor, and the evaporation unit takes the drain waste liquid that has been treated in the separation unit and evaporates it inside an exhaust duct used for the cooling air of the air compressor.
An adsorption type of apparatus has been disclosed in JP2004-154630 (A), in which a vessel is packed with an FRP powder, and oil is removed by being adsorbed to the surface of this FRP powder.
An electrolysis type of apparatus is disclosed in Japanese Patent 2,691,119, in which excess metal ions are eluted from an electrode, thereby producing a floc of aluminum hydroxide or the like, the cohesive strength of this aluminum hydroxide is utilized to produce clumps of colloidal oil, and these are recovered by gravitational separation or adsorptive separation, thereby removing the oil.
In the prior art discussed above, however, with a filtration membrane type the membrane itself is expensive, which drives up the running cost of the apparatus. Furthermore, because the pores in the filtration membrane are so small, they tend to become clogged, requiring washing or other such treatment, which is a drawback in that the treatment efficiency is low.
A drawback to a specific gravity separation type of apparatus is that the state of separation can be uncertain because of the small difference in specific gravity between water and oil. Consequently, a multi-step treatment is necessary, but this results in a larger-scale apparatus configuration, and if the oil is in the form of an emulsion, some of the oil may not be separated, among other drawbacks.
An advantage to an adsorption type of apparatus is that if the oil is dispersed in the form of relatively large particles in water, the oil can be removed at high precision, and treatment efficiency is high. Nevertheless, when the oil is in the form of a fine colloidal emulsion, the percentage of oil removed decreases and treatment performance is low.
With an electrolytic apparatus, the separated aluminum hydroxide floc (hydrous sludge) has to be treated as industrial waste, the apparatus configuration becomes larger, and the cost is higher. Also, when metal ions are used in a large quantity, the positive electrode is consumed more quickly, shortening the service life of the electrode, and electrical power consumption also rises.
There are no restrictions on the oil-containing mixture to be treated here, which can be any liquid containing waste oil from some kind of machinery. In particular, though, the waste oily water from a compressor drain contains oil in a concentration of 150 to 500 ppm, and the particle size of this oil can be anywhere from about 0.001 to 100 μm. Accordingly, oil with a small particle size, and particularly colloidal oil, can pass right through the oil adsorbent layers of a conventional apparatus, making it difficult to thoroughly separate all of the oil.